Since I went from my stock 15 tires to 235/40/18 tires my MPG has dropped significantly from about 26mpg to about 20-21mpg. Is this normal. Has anyone else experienced this...if so why does this happen. Thanks.

 

I'd be willing to bet it's the rims, not the tires. With the heavier 18's the car will need to work harder to get them rolling and keep them rolling.

 

Possibly what you've encountered is a simple gear change. The larger diameter of your new rims has effectivley changed your final drive ratio. So your wheels are covering more ground with each revolution and your odometer is reading differently becouse of this. Picture one of those Mountain Bikes with the multi cog gearsets on them. When you change the rear derailer to a larger diameter (bigger) gear it becomes harder for you to peddle from a standing start. Since your cars odometer is geared to the transmission it turns at a fixed rate. Your wheel change has altered the gear ratio. I hope this reply helps.

 

The gear ratio can cause a slight decrease in MPG. However, that is a huge drop. I have 18's on my car and i still run 28 mpg when im not beating on it. The weight of the rim will effect you with back roads and when accelerating but not once you are at a constant speed as much. Usually people find that with new mods they now drive the car harder (around turns) and that is why the MPG decreases. That would be my guess.

 

I think that is a bit too significant of a gas milage decrease. It would be more logical with a 1-2 mpg drop.

It definately takes more work for the car to get going with the added rotational mass, so a little milage decrease is sacrifice you make with bigger/heavier wheels.

 

Gearing should not be affected as the tires are the same overall diameter. I'd check the alignment and maybe think about the increased rotational mass (more weight is concentrated further out on the wheel now too), and the fact that you now have wider, high performance tires that probably have a lot more rolling resistance. I have no clue if you should be losing that many MPG though.... *shrug*

 

Wheel weight (and the added gas pedal pressure from them slowing you down) and wider tires are probably the biggest killers. Also make sure you have enough tire pressure, you should probably be running 2+ psi over stock which should put you around 35/32 front/rear. Tire pressure makes a big difference in mileage.